For many years analytical chemists did not use Raman spectroscopy because it failed to provide the degree of sensitivity required for the detection of trace quantities of analytes. The main reason for this is the high background levels of fluorescence arising from either the sample or substrate.
In 1974, Fleischman discovered, whilst using Raman spectroscopy to study the electrochemical reactions of pyridine on a silver electrode, that there was a remarkable enhancement of the pyridine Raman signals, with the silver quenching a large amount of the background fluorescence. It is known that surface enhancement could only be achieved if the silver surface was rough and not smooth.
The possibility of utilizing colloidal dispersions of either silver (Ag) or gold (Au) in aqueous solutions was first demonstrated by Creighton and co-workers in 1979 (Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J Chem. Soc., Faraday Trans. 2 1979, 75, 790). It has been found that equal or even higher surface enhancement effects can be achieved with silver colloids. A colloid is a suspension of the metal particles in solution. In order to achieve the optimum effect, controlled aggregation of the silver colloid particles is required, typically using organic or inorganic compounds as aggregation reagents.
With the tremendous increase in sensitivity that can be achieved using this surface enhancement effect, the analytical techniques of Surface Enhanced Raman Scattering (SERS) Spectroscopy and Surface Enhanced Resonance Raman Scattering (SERRS) Spectroscopy have since been developed.
The growth in the use of these techniques has been exponential but the major problem is producing stable colloids with good light scattering properties and capable of quenching background fluorescence. In order for a colloid to remain stable the silver particles should remain suspended indefinitely, but it is known that on many occasions aggregation occurs and the silver falls out of solution.
Silver colloids can be prepared by chemical reduction with either sodium borohydride or sodium citrate. It is well known that citrate reduced colloids are more stable and many analysts have prepared these using a method published by P. C. Lee and O. Meisel (J. Phys. Chem., 1982, 86, 3391-3395). However, it is well known that batch-to-batch reproducibility is difficult to achieve by this method and the stability, i.e. shelf life, is variable. Preparation of such silver colloids using this method requires the use of ultra clean glassware and accurately controlled temperatures, stirring speed, etc.
Since this original published method there has been a published modification of this original method (C. H. Munro, W. E. Smith and P. C. White, Analyst 1993, Vol. 118. 733-735). This published modification of the original known method led to some improvements in the properties of the silver colloid but long term stability of the colloids still remained a problem.
Prior attempts at producing silver colloids with desirable light scattering properties have been poor and because of these disappointing results there has been little encouragement to go against the perceived wisdom that the ionic nature of sodium in silver nitrate was in fact responsible for the failed attempts at obtaining silver colloids with suitable stability and shelf-life. In WO2007/107792 we disclose and claim a method of producing very stable silver colloids with good SERRS properties using lithium citrate instead of sodium citrate to reduce silver nitrate.
The use of hydroxylamine hydrochloride to reduce silver nitrate at alkaline pH and at room temperature was published in Leopold, N.; Lendl B.; J. Phys, Chem. B 2003, 107, 5723-5727. The results published in this paper show that the colloids have a high bandwidth and λmax values with a large particle size distribution. These properties are typical of poor colloids and would not be expected to give good SERRS spectra. The spectra presented show high levels of fluorescence background, which is typical of poor adsorption of the dye on to the silver particles. There is no mention in the paper of the stability or reproducibility of SERRS spectra from different batches of colloid, and this has been the major problem in SERRS spectroscopy.
Gold colloids can be used in various nanotechnology applications, for example in biosensors, as well as in Raman Spectroscopy. Other metals in colloidal form may be applicable to similar applications, or may find new applications.